The invention relates to a unitary lithium secondary battery having a laminated electrode and separator membrane structure. More particularly, the invention further relates to a method of extracting plasticizer from the structure to provide a battery which may be stored for an extended period of time without the need for anhydrous conditions since no electrolyte solution, i.e., lithium salt, is present.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on porous separator structures (fiber mats) or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solution of a lithium compound, such as LiClO.sub.4, LiPF.sub.6 or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or ethylene carbonate or their mixtures, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li.sup.+ ion mobility at conductivities in the range of about 10.sup.31 3 S/cm.
By contrast, in the present invention, Li ion mobility is provided through the polymeric network which makes up not only the separator but also the electrodes of the layered unitary battery structure of the present invention.
Although prior art separator membranes were often formed of polymeric materials, prior art battery structures of discrete electrodes and separator membranes had numerous disadvantages over the unitary battery structures of the present invention. One prior art separator membrane, as disclosed in U.S. Pat. No. 3,351,495, is composed of a polyolefin material which may contain a plasticizer and a filler. When the polyolefin is mixed with a plasticizer and a filler material, the mixture is extruded to form a sheet and then a sufficient amount of plasticizer is removed to provide a finished separator membrane. The removal of the plasticizer is necessary to provide a void volume within the polyolefin which creates a microporous structure for introduction of the electrolytic fluid.
Although serving well in this role of ion conductor, these separator elements unfortunately comprise sufficiently large solution-containing voids that continuous avenues may be established between the electrodes, thereby enabling lithium dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated manganese oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Although providing efficient power sources, these lithium-ion cells do not readily attain the capacity provided by lithium metal electrodes.
Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO.sub.4, LiPF.sub.6, or the like. A range of practical ionic conductivity, i.e., over about 10.sup.-5 to 10.sup.-3 S/cm, was however, only attainable with these polymers at ambient conditions well above room temperature. Some improvement in the conductivity of the more popular polyethylene oxide (PEO) compositions has been reported to have been achieved by controlling the high temperature mechanical stresses of the PEO by radiation-induced crosslinking (U.S. Pat. No. 5,009,970) or by meticulous blending with exotic ion-solvating polymer compositions (U.S. Pat. No. 5,041,346). Each of these attempts achieved limited success due to attendant expense and restricted implementation in commercial practice.
An example of the foregoing has been the use of polymer host network electrolytes as described, for example, in U.S. Pat. Nos. 5,219,680 ; 4,925,752; and 4,303,748. The advantage of these polymer host networks is the ability to form battery structures which are small and have good conductivities. Like the present invention, these solid state electrolytes may be useful in forming unitary solid state batteries. The solid state batteries described in these patents are, however, water-sensitive and must be protected from moisture during production and post-production storage. These battery elements, i.e., electrodes and separators, are produced by the inclusion of the electrolytic salt as a component of the polymer material prior to forming of the elements.
The present invention provides a means for avoiding the disadvantages of prior electrolytic cell compositions and constructions by enabling the ready and economical preparation of strong, flexible unitary laminated battery cells which will readily retain electrolyte salt solutions and remain functional over a range extending well below room temperature, as well as avoiding water sensitivity and providing storage stability in an uncontrolled environment over a long period of time.